Ocean Carbon Sequestration
The Deep in Deep Blue Carbon
Ocean based negative carbon has another important advantage over land-based techniques: A VERY easy way to sequester the carbon safely for a LONG time.
The ocean is DEEP (see for yourself with this interactive map). Most places are well over 3 KM deep. At this depth there is very little oxygen, so decomposition happens very slowly, but more importantly, at depths greater than 3KM, the pressure is so great that CO2 is a liquid that is denser than seawater. Thus if it will sink to the bottom (if it is not already there). The water is also cold (about 4 degrees C) and at that temperature and pressure CO2 forms hydrates (a slush) which is also denser than seawater. Thus any biomass that is dropped in the ocean will sink to the bottom and the carbon will simply stay there.
There are VERY slow currents (see Thermohaline circulation) but they take hundreds of years to surface, and are not likely to dredge up heavy solids from the bottom. Thus the CO2 should stay sequestered for at least 100s of years.
This gives us a very simple system for sequestering carbon. Grow seaweed in the open ocean and let some of its biomass sink to the bottom. The carbon associated with that biomass will not reenter the atmosphere for a very long time. This is about as simple as you could imagine carbon sequestration getting.